Polyolefins, such as polyethylene (PE), is synthesized by polymerizing olefin, such as ethylene (CH2═CH2), monomers. Because it is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the properties polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
Olefin polymerizations are frequently carried out in a loop reactor using monomer, liquid diluent and catalyst, optionally one or more co-monomer(s), and hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash vessel, where most of the diluent and unreacted monomers are flashed off and recycled.
Alternatively, the product slurry may be fed to a second loop reactor serially connected to the first loop reactor wherein a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product is a bimodal polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, and has a bimodal molecular weight distribution.
After the polymer product is collected from the reactor and the hydrocarbon residues are removed, the polymer product is dried, additives can be added and finally the polymer may be extruded and pelletized.
During the extrusion process ingredients including polymer product, optional additives, etc, are mixed intimately in order to obtain a compound as homogeneous as possible. Usually, this mixing is done in an extruder wherein the ingredients are mixed together and the polymer product and optionally some of the additives are melted so that intimate mixing can occur. The melt is then extruded into a rod, cooled and granulated, e.g. to form pellets. After extrusion, the obtained polymer pellets are further processed in a pellet treatment unit. In this form the resulting compound can then be used for the manufacturing of different objects.
When consecutively treating different polymer pellets in a same pellet treatment unit, adequate cleaning of the unit is required in between the different operations in order to remove any polymer material from previous runs remaining in any elements of the pellet treatment unit. Polymer residues from a first grade can contaminate subsequently produced polymer grades. Cleaning steps can maintain the quality of the subsequently produced polyethylene batch but are demanding in terms of labour, increase production costs and/or lead to longer down-time.
In view of the above, the present inventors have identified a need in the art for an improved process for the consecutive production of different olefin polymers.